How to copy field and value within a document to another document in a different collection in mongo-CodePudding

I have a collection called X which holds a single document that looks like this:

{
    "uid": "dfdf"
}

I have another collection called Y which also holds a single document that looks like this:

{
    "name": "bfbf"
}

I want to add to the document from collection Y, the field and value from collection X so it will look like this:

   {
       "uid": "dfdf",        
       "name": "bfbf"
   }

I ran the next code on the Y collection:

{$set : {"uid" : {"$query": {"$q": {}, "$ns": "**X**", "$p": "uid"}}}}

BUT the value is set with array like this:

   {
       "uid": ["dfdf"],        
       "name": "bfbf"
   }

How can I set the original value without the array? Thanks in advance

CodePudding user response：

It seems that OP simply want to do uncontional lookup and assign the field uid into the collection Y

db.Y.aggregate([
  {
    "$lookup": {
      "from": "X",
      "pipeline": [],
      "as": "x"
    }
  },
  {
    "$unwind": "$x"
  },
  {
    $set: {
      "uid": "$x.uid"
    }
  },
  {
    "$unset": "x"
  }
]).forEach(function(doc){
  db.Y.update({_id: doc._id}, {$set: {uid: doc.uid}});
});

Here is the Mongo playground for your reference.